This invention relates to improvements in load-handling implements for lift trucks wherein forwardly-protruding, selectively openable and closable opposed clamp arms are attached to the ends of transversely-oriented, longitudinally-movable elongate slide members mounted on a slide guide frame attached to a lift truck load carriage.
Stress concentrations are responsible for a high incidence of fatigue failures at the joint between the rear of such a forwardly-protruding clamp arm and the end of the respective slide member upon which the load arm is mounted. Such failures result from the cyclic application of primarily horizontal forces to the joint, caused by the load moment and clamping moment respectively. The load moment is the reactive moment exerted about a generally horizontal axis transverse of the lift truck by the weight of the forwardly-protruding load, causing the clamp arm to pull forwardly away from the truck at its top and push rearwardly toward the truck at its bottom. The clamping moment is the reactive moment exerted about a generally vertical axis by the clamping pressure on the load which tends to pivot the clamp arm laterally outward with respect to the end of the respective slide member upon which it is mounted.
In conventional structures utilizing tube-type slide guides, as shown for example in Ehmann U.S. Pat. No. Re. 23,694 and Saint U.S. Pat. No. 2,821,316, each slide member includes a metal connecting member of small cross section and length, compared to the cross section and length of the remainder of the slide member, welded to one end of the slide member and protruding forwardly therefrom attaching at its forward end to the rear of a clamp arm. The relatively small cross section and length, and the forward protrusion, of the welded connecting member is necessary because such member must communicate between the interior and the exterior of the tube-type slide guide through a narrow slot in the guide when the clamp arms are closed to a narrow position within the width of the clamp guide frame. This type of construction, which is also utilized in structures having tube-type guides of square or rectangular shape as shown for example in U.S. Pat. Nos. 2,609,114 and 2,746,630, introduces a high degree of susceptibility to failure from the above-described cyclically-applied horizontal forces at the welded joint because the protruding connecting member at the end of each slide member causes a relatively abrupt cross-sectional change horizontally in the slide member at the point where the weld occurs, thereby creating a stress concentration which is further aggravated by the lack of homogeneity in the metal caused by the weld at that point.
The above-described susceptibility to fatigue failure has been remedied in some previous clamps by providing large horizontally-extending gussets at the juncture between the slide member and clamp arm, as shown for example in U.S. Pat. Nos. 2,635,774, 2,870,929 and 2,956,700. However such gussets interfere with the handling of a rectangular load in the optimum position where the rear surface of the load is closely adjacent to the slide members. Attempting to hold a rectangular load in a more forward position where it does not interfere with the gussets is unsatisfactory since the greater forward tipping moment thus exerted by the load on the lift truck greatly reduces the load-carrying capacity and stability of the truck.
Still other clamp constructions have solved the aforementioned failure problem by providing slide members of substantially uniform horizontal cross-sectional dimension throughout their length, each having an elongate portion along the front thereof which is exterior of the slide guide to which the load arm may attach directly without the necessity of any small, welded, protruding connecting member. In the most pertinent previous construction of this type, which is disclosed in U.S. patent application Ser. No. 920,455, filed June 29, 1978, the clamp arm is bolted or welded directly to the front face of the elongate exterior portion of the slide member at one end thereof. This results in an extremely failure-resistant connection between the clamp arm and the slide member, provided that the rear end of the clamp arm where it connects to the face of the exterior portion of the slide member has substantial transverse thickness.
In some materials handling applications, however, it is impossible for clamp arms to have such substantial thickness at their rear ends. This is particularly true in cases where cartons or similar loads are stored with little space between them for insertion of the clamp arms. In such cases the clamp arms must be relatively narrow throughout their lengths as, for example, shown in Lord U.S. Pat. No. 2,782,065. However to accommodate such narrow clamp arms a plate extending inwardly perpendicularly from the rear end of each clamp arm must be provided, as in Lord, for mounting upon the face of the respective slide member at one end thereof. These plates, which must be of substantial thickness to resist the stresses imposed on the clamp arms, unfortunately prevent loads from being handled in a position abutting the face of the slide members. Rather each load must be handled in a forwardly-protruding position with its rear surface pushed forwardly from the face of the slide member and separated therefrom by the thickness of the aforementioned plate. Since most lift trucks which would utilize such a clamp are of the counterbalanced type, the front axle thereof serving as a fulcrum, any requirement that the load be positioned a spaced distance forwardly of the face of the slide members necessarily diminishes the counterbalanced load-carrying capacity of the lift truck because the forward distance between the center of gravity of the load and the front axle of the truck is thereby increased.
It should also be noted that the aforementioned plates cannot extend outwardly instead of inwardly from the clamp arms to eliminate the problem of limited rearward load position, since such plate would then merely strike the front surfaces of loads positioned on either side of the load being engaged during insertion of the clamp arms, thereby likewise limiting the rearward position of the load relative to the clamp arms.
Moreover the plates which cause such forward positioning of the load in applications requiring narrow clamp arms cannot be eliminated by dispensing with the plate and merely welding or bolting the rear ends of the narrow clamp arms at right angles to the ends of the slide members, since such right-angle joint is the point of maximum stress concentration imposed by the aforementioned clamping moments and any lack of homogeneity of material at this joint, caused by welding or bolting, would severely aggravate the stress concentration which would soon lead to fatigue failure.
Accordingly what is needed is a structure for mounting narrow clamp arms to slide members of the type having stress-resistant elongate front portions exterior of the slide guides, such clamp arm attaching structure permitting placement of the load in abutment with the exterior front portion of the slide member to maximize rearward positioning thereof and thus maximize load-carrying capacity of the truck, such clamp arm attaching structure also providing homogeneity of material at the right-angle junction between the clamp arm and slide member.